The cooling system or the circulating water system provides a continuous supply of cooling water to the main condenser to remove the heat rejected by the turbine and auxiliary systems (e.g. the turbine bypass system). In this process the cooling water becomes hot. This energy is rejected to the atmosphere via cooling towers or directly to the seawater or to a river. Note that, not all nuclear power plants have cooling towers and conversely, the same kind of cooling towers are often used at large coal-fired power plants as well.
Cooling System in Wet Steam Turbines
In a typical condensing steam turbine, the exhausted steam condenses in the condenser and it is at a pressure well below atmospheric (absolute pressure of 0.008 MPa, which corresponds to 41.5°C). This steam is in a partially condensed state (point F), typically of a quality near 90%.
Rankine cycle – Ts diagram
The pressure inside condenser (thus the enthalpy of outlet steam) is given by the ambient air temperature (i.e. temperature of water in the cooling system). and other parameters:
- air temperature, pressure and humidity in case of cooling into the atmosphere
- water temperature and the flow rate in case of cooling into a river or sea
An increase in the ambient temperature may cause a proportional increase in pressure of exhausted steam (ΔT = 14°C is usually a constant) hence the thermal efficiency of the power conversion system may decrease. In other words, the electrical output of a power plant may vary with ambient conditions, while the thermal power remains constant.
It must be noted there is also lower limit for the steam outlet temperature, thus for the cooling system. Below 0.008 MPa and 41.5 °C the specific volume of exhausted steam significantly increases which requires huge blades in last rows of low-pressure stage of the steam turbine. Moreover, with a decrease in the turbine exhaust pressure the vapor quality decreases (or dryness fraction). At some point the expansion must be ended to avoid damages that could be caused to blades of steam turbine by low quality steam.
Typical parameters in a condenser of condensing turbines
In modern nuclear power plants the overall thermal efficiency is about one-third (33%), so 3000 MWth of thermal power from the fission reaction is needed to generate 1000 MWe of electrical power. Therefore about 2000MWth of unusable energy must be rejected in order to fulfill the second law of thermodynamics.
To maintain the parameters inside the condenser (0.008 MPa and 41.5 °C), the cooling water must be sufficiently cold and there cannot be large temperature difference between the outlet and inlet water temperauter, hence the flowrate through the cooling system must be very high. The flowrate through the cooling system (with wet cooling towers) may be up to 100 000 m3/h (27.7 m3/s). The condenser inlet water may have about 22°C (strongly depending on ambient conditions), while the condenser outlet may have about 25°C.
The sea water cooling systems operate at higher flowrates, for example, 130 000 m3/h. In general, the energy can be rejected to the atmosphere via cooling towers or directly to the seawater or to a river.